1. Field of the Invention
This invention is directed to an apparatus and to a method. More specifically, this invention concerns a multi-channel ventilation monitor and a method for real time monitoring of multiple signals associated with breathing and cardiovascular activity.
2. Description of the Prior Art
The monitoring of physiological activity of individuals with various types of instrumentation is an accepted method used by clinicians to detect abnormalities in biological functions. Monitors are available to measure cardiovascular activity, respiratory activity, and central nervous system activity. In the monitoring of physiological activity, a sensor is generally affixed to the individual and, in turn, connected to a signal interpreter. The signal interpreter can be relatively simple (i.e. U.S. Pat. No. 4,169,462--to Strube), or involve relatively simple and unsophisticated information processing (i.e. U.S. Pat. Nos. 4,306,567--to Krasner; and 4,356,825--to Veth).
In the device described by Strube, an electromechanical transducer, incorporating a piezoelectric crystal, is attached to the monitored subject. The transducer is used to detect variation in mechanical pressure resulting from breathing activity. Upon detection of such motion, the transducer relays an impulse, in the form of an electrical signal, to a counting circuit. An oscillator is associated with the counting circuit which in turn is preset to a predetermined counting period. The oscillator simply feeds a repetitive signal into the counter. If the transducer fails to generate a signal within a predetermined counting period to reset the counter, a switch is closed by the counting circuit which in turn triggers the alarm.
In the device described by Krasner, a sensor is attached to the monitored subject which is sensitive to acoustical signals indicative of a physiological rhythmic function. The Krasner device is designed to monitor acoustical signals within a relatively narrow frequency. These signals result from mechanical displacement of the body beneath the sensor. The design of the sensor and its attachment directly to the skin of the subject, is reputed to reduce the amount of spurious and environmental noise, in an attempt at enhancement of the signal-to-noise ratio. The electrical signal from the sensor is demodulated to enable detection of the periodic amplitude and modulated frequencies of the electrical signal. Once the signal has been reshaped or standardized, it is further processed to eliminate artifact within the predetermined acoustical frequency of interest. This further processing involves comparison of the duration of the demodulated signal, within the frequency bandwidth of interest, to a function rate signal. The Krasner monitor is designed to reject any signal within the frequency bandwidth of interest if it is less than or in excess of a predetermined signal duration (signals of less than 400 milliseconds duration and longer than 3-4 seconds being rejected as attributable to artifact). The relatively simplistic information processing logic of Krasner is thus unable to discriminate between true artifact and respiratory signals which may be outside the system parameters.
In the device described by Veth, a series of sensors are attached to the monitored patient for detection of different physiological activities (i.e. pulse rate, respiration and temperature). The data collected from these sensors is interpreted by a digital microprocessor associated with this electrical measuring system. This device is capable of intermittent measurement and processing of the input signal from only a single activity at any one time. The input signal is compared to a system clock, which utilizes a conventional oscillator (with an adjustable output frequency) as a standard. As is evident from the emphasis by Veth on rapid data analysis, his device is directed to an intermittent data sampling system. More specifically, the detection of successive data points is based upon a data sampling scheme involving the collection of a limited number of samples; the sampling interval and count frequency being controlled by a voltage controlled oscillator output signal. Accordingly, the Veth device is designed to restrict data sampling to a relatively brief period; with the data points being selected in this preselected interval and the interval being controlled by the voltage control oscillator.
The devices described hereinabove are fairly typical of those presently commercially available. Each of the above devices described by Krasner and Veth, respectively, share certain basic information processing similarities, (i.e. root mean square processing of signal data) as well as lack the ability to discriminate between apnea which is associated with obstruction of the breathing passageway from apnea which originates within the central nervous system (CNS). In addition, each of the foregoing devices lacks the ability to "effectively" distinguish between normal respiration and respiratory activity which may be indicative of an apnea episode. The reason for this deficiency is the inability of these systems to dynamically accommodate, or adjust, themselves to changes in the body position or the level of respiratory activity of the monitored subject. Thus, when the monitored subject is awake at the time monitoring commences, or is sleeping and progressively goes into a deeper sleep state, the level of physiological activity of the patient will change and the relative frequency of the signal will also change accordingly. When the sensitivity of the instrument is set to accommodate the intial activity state, it cannot effectively monitor the patient as the activity state becomes progressively more shallow. Attempts at overcoming this deficiency by setting the initial sensitivity at a very high level results in frequent false alarms and, thus, tends to impair the credibility of the monitor. The individual responsible for responding to such an alarm may not react quickly enough to intervene in a real emergency because of the skepticism which is created due to numerous false alarms. Conversely, where the sensitivity is set to accommodate the initial activity state, the monitor will be unable to effectively identify a true apnea episode requiring intervention as the physiological activity level becomes more shallow. Thus, there is a continuing need for improvement in respiration and ventilator monitors to reduce or eliminate the frequency of false alarms and yet provide effective monitoring of the patient at various and ever changing levels of respiratory activity.